1. Field of the Invention
The present invention is directed to a reactor configuration especially useful in heterogenous reactions employing a solid catalyst in a liquid phase, and to an olefin epoxidation process employing such a reactor.
2. Background Art
The majority of propylene oxide produced today is produced by so-called “coproduct” processes in which an easily oxidizable substrate is oxidized to produce hydroperoxides and/or peroxides, which are then used to “indirectly” oxidize propylene. The reduction products of the oxidized substrate are generated in large quantities and sold as coproducts. Typical coproducts are styrene and methyl-t-butylether. Since the process necessitates purchase of the oxidizable substrates and sale of coproduct, the price of each of which may vary widely, “direct oxidation” processes have been sought wherein market fluctuations do not dictate the overall economy of the process.
While epoxidation of ethylene with oxygen over a supported silver catalyst has been widely used, an analogous oxidation of propylene is not viable. Recent research activity directed to “direct oxidation” of propylene has concentrated on use of hydrogen peroxide, generated externally or in situ, in the presence of titanium silicate zeolites such as titanium silicalite as catalysts. The solid crystalline catalyst particles may be treated to contain a noble metal which catalyzes hydrogen peroxide production from hydrogen and oxygen. If such a process could be commercially practiced, only low cost reactants would be used, and no coproduct produced.
In U.S. Pat. No. 6,376,686, an olefin epoxidation process is described employing a solid catalyst in a reactor configuration similar to that disclosed in U.S. Pat. No. 5,972,661, herein incorporated by reference. In the latter patent, a “draught tube” reactor is disclosed, a simplified schematic of which is illustrated by FIG. 1. The reactant slurry, including solid catalyst, enters the central draught tube and is directed along the axis of the draught tube by impeller(s) therein. A series of vertically oriented baffles positioned between impellers prevents a swirling flow which might cause solid catalyst segregation. Upon reaching the end of the draught tube, the slurry flows in a countercurrent direction through an annulus between the draught tube and the reactor wall. Product is continuously removed, separated from entrained catalyst, and worked up to remove solvent, byproducts, etc. Catalyst must be returned to the reactor. The reactor configuration has been found to exhibit high mass transfer and mixing rates. The catalyst is a combination of active ingredient and inert binder. Under intense agitation, the catalyst can gradually undergo break-up producing fines that lead to pluggage of filtration equipment. The fines could also pass through the filtration equipment resulting in catalyst loss from the reactor-filter setup.
It would be desirable to produce a reactor configuration which can take advantage of the mass transfer characteristics of a draught tube reactor while causing less catalyst attrition.